Selenium Properties and Toxicity
Selenium (element Se) is a metalloid that can form organic and inorganic compounds and exist in a number of oxidation states (i.e., II-, 0, II, IV, and VI; although Se[II] is not found in nature). Common selenium containing molecules (and their oxidation states) are Selenium (Seo), selenite (Se[IV], SeO32−), selenate (Se[VI], SeO42−), and hydrogen selenide (Se[II-], H2Se).
Selenium is used in the production of meals, glass, pigments, textiles rubber, metal alloys, textiles, petroleum products, photoelectric applications, etc. Selenium was used is pesticides, although use has been curtailed. The toxicity of selenium to mammals and birds is well known. Selenium is required as a trace element in mammals but is toxic in elevated doses. Selenium contamination is ubiquitous in many parts of the world, including the western United States.
Selenium is sufficiently similar in bonding characteristics and size to substitute for sulphur in various metabolic reactions. When incorporated into bacterial sulphur metabolism, selenium can become bonded to carbon and incorporated into selenoamine, a biologically active amino acid. Selenoamine is incorporated into selenoproteins. This selenium can reappear in many “sulphur”-containing biological molecules, including selenocysteine and selenomethionine, which are toxic analogs of cysteine and methionine, respectively. Dimethyl selenide (DMSe, (CH3)2Se) and dimethyl diselenide (DMDSe, (CH3)2Se2) are volatile forms of selenium also believed to be the result of microorganism metabolism.
Oxidized forms of selenium exist as soluble oxyanions in water. Selenate is predominant in freshwater and selenite is predominant in seawater. Neither of these anions reacts appreciably with calcium or magnesium to form a precipitate. Elemental selenium is found in amorphous form (crimson red) or in one of several crystalline forms, which are less colorful. Reduced selenium species include hydrogen selenide (H2Se), methylselenides, and others.
Crude oils often contain high levels of selenium, particularly when associated with geological formations containing marine shales. During refinement, selenium becomes concentrated in refinery effluents in the forms of selenide, selenite, selenate and selenocyanate. The presence of hydrocarbons and other petroleum products complicates selenium removal.
Removal of Selenium from Water
Conventional methods for removing selenium contaminants from water include chemical addition followed by precipitation or adsorption to a solid phase or membrane filtration to separate the oxidized or reduced selenium species. For example, U.S. Pat. No. 4,915,928 describes a process for removing selenium from wastewater using a strong ion exchange resin. U.S. Pat. Nos. 5,993,667 and 6,156,191 described a process for removing selenium from wastewater involving oxidizing seleniumcyanate ions to selenite ions, which are removed by adsorption to ferric hydroxide and other insoluble precipitates. WO 92/07798 describes a method for removing selenium from an aqueous stream using a particular chemical oxidizing agent. U.S. Pub. No. 2005/0079114 describes a method for removing selenium from an aqueous stream using a quaternary ammonium compounds. Generally, zeolites, dithiocarbamates (and polymers, thereof), metal salts (e.g., ferric chloride and ferric sulfate) adsorb selenium is in the form of selenite anions but are not effective in removing selenocyanate ion (SeCN−), which predominate in refinery wastewater and stripped sour water.
Air, ozone, hydrogen peroxide, and chlorine dioxide have been used as oxidizing agents; however, air is ineffective in oxidizing selenocyanate, ozone and hydrogen peroxide conventionally require highly alkaline conditions for maximum effectiveness, and chlorine dioxide is unstable and introduces chloride to the water. While the reduction of selenate (VI) to selenite (IV) is thermodynamically favorable it requires a strong reductant to speed the reaction. Notably, some bacteria are able to reduce selenium metabolically using hydrogen or another reducing gas, which avoids the need for a strong reductant.
Conventional methods for removing selenium from wastewater are not particularly effective in water with high total dissolved solids (TDS), particularly salts. These waters include brine water, which may have well over 1,000 parts per million (ppm) salt.
The need exists for an economical and efficient water treatment process that can remove selenium species from high salt/high TDS water and refinery wastewater.